Currently, the lens is widely used in illuminating device for realizing various illumination effects and improving illumination efficiency. The lens should be precisely located above a corresponding light source or to cover the light source. In various current illuminating devices, the lens is usually fixed on a circuit board carrying a light source by means of a mechanical fixing element, such as screw or bolt. Especially when a part having a lens structure is used as a housing of the illuminating device, in order not to affect the illumination effect, the lens structure is only arranged in center of the housing, and the lens structure is fixed on the circuit board at an edge region of the housing by means of a mechanical element.
Since only a pressure from the mechanical fixing element is received at the edge region, only the edge region of the housing can be assured to be pressed against the circuit board in such a manner, while a central region might be deformed or warped with respect to the edge region. Such deformed or warped situation is more prominent upon influence of external environment. For instance, an outdoor illuminating device should be subject to certain temperature difference change, for example, a housing made of plastic and having a lens structure expands with heat and contracts with cold as temperature changes. When it expands with heat, although only the edge region of the housing is still fixed on the circuit board, the central region thereof deforms and arches upward with respect to the circuit board. This causes change to a corresponding relation between the lens structure and the light source and therefore will seriously affect the illumination effect of the illuminating device.